1. Field of the Invention
The present invention relates to a projection apparatus implemented with a laser light source for emitting an illumination light modulated by a spatial light modulator to project a modulated light to display an image. More particularly this invention relates to an image position change unit to change the image projection positions within a predefined range to reduce the occurrence of the speckle effect.
2. Description of the Related Art
After the dominance of CRT technology in the display industry for over 100 years, Flat Panel Displays (hereafter FPD) and Projection Displays have gained popularity because the FPD display implements a more compact image projecting system while projecting images on a larger display screen. Of several types of projection displays, projection displays using micro-displays are gaining recognition among the consumers because of their high picture quality and a lower cost than FPDs. There are two types of micro-displays used for projection displays on the market, i.e., micro-LCDs (Liquid Crystal Displays) and micromirror technology. Because the micromirror devices display images with an un-polarized light, the images projected by the micromirror device have a brightness superior to that of micro-LCDs, which use polarized light.
Even though there have been significant advances made in recent years in the technologies of implementing electromechanical micromirror devices as spatial light modulators (SLM), there are still limitations and difficulties when they are employed to display high quality images. Specifically, when the display images are digitally controlled, the quality of the images is adversely affected because the images are not displayed with a sufficient number of gray scale gradations.
Electromechanical micromirror devices have drawn considerable interest because of their application as spatial light modulators (SLMs). A spatial light modulator requires an array of a relatively large number of micromirror devices. In general, the number of devices required ranges from 60,000 to several million for each SLM. Referring to FIG. 1A for a digital video system 1 includes a display screen 2 disclosed in a relevant U.S. Pat. No. 5,214,420. A light source 10 is used to generate light beams to project illumination for the display images on the display screen 2. The light 9 projected from the light source transmitted through the mirror 11 is further collimated and directed toward lens 12. A beam columnator includes lenses 12, 13 and 14 is operative to columnate the light 9 into a column of light 8. A spatial light modulator 15 is controlled by a computer through data transmitted over data cable 18 to selectively redirect a portion of the light from path 7 toward lens 5 to display on screen 2. FIG. 1B shows a SLM 15 that has a surface 16 that includes an array of switchable reflective elements 17, 27, 37, and 47, each of these reflective elements is attached to a hinge 30. When the element 17 is in an ON position, a portion of the light from path 7 is reflected and redirected along path 6 to lens 5 where it is enlarged or spread along path 4 to impinge on the display screen 2 to form an illuminated pixel 3. When the element 17 is in an OFF position, the light is reflected away from the display screen 2 and, hence, pixel 3 is dark.
Most of the conventional image display devices, such as the devices disclosed in U.S. Pat. No. 5,214,420, are implemented with a dual-state mirror control that controls the mirrors to operate in either an ON or OFF state. The quality of an image display is limited due to the limited number of gray scale gradations. Specifically, in a conventional control circuit that applies a PWM (Pulse Width Modulation), the quality of the image is limited by the LSB (least significant bit) or the least pulse width, since the control is related to either the ON or OFF state. Since the mirror is controlled to operate in either an ON or OFF state, the conventional image display apparatuses have no way of providing a pulse width to control the mirror that is shorter than the LSB. The lowest intensity of light, which determines the smallest gradation to which brightness can be adjusted when adjusting the gray scale, is the light reflected during the period corresponding to the smallest pulse width. The limited gray scale gradation due to the LSB limitation leads to a degradation of the quality of the display image.
In FIG. 1C, a circuit diagram of a control circuit for a micro-mirror according to U.S. Pat. No. 5,285,407 is presented. The control circuit includes memory cell 32. Various transistors are referred to as “M*” where * designates a transistor number and each transistor is an insulated gate field effect transistor. Transistors M5, and M7 are p-channel transistors; transistors, M6, M8, and M9 are n-channel transistors. The capacitances, C1 and C2, represent the capacitive loads presented to memory cell 32. Memory cell 32 includes an access switch transistor M9 and a latch 32a, which is the basis of the Static Random Access switch Memory (SRAM) design. All access transistors M9 in a row receive a DATA signal from a different bit-line 31a. The particular memory cell 32 to be written is accessed by turning on the appropriate row select transistor M9, using the ROW signal functioning as a word-line. Latch 32a is formed from two cross-coupled inverters, M5/M6 and M7/M8, which permit two stable states. State 1 is Node A high and Node B low and state 2 is Node A low and Node B high.
The control circuit, as illustrated in FIG. 1C, controls the micromirrors to switch between two states, and the control circuit drives the mirror to oscillate to either an ON or OFF deflection angle (or position) as shown in FIG. 1A. The minimum intensity of light controllable to reflect from each mirror element for image display, i.e., the resolution of gray scale of image display for a digitally controlled image display apparatus, is determined by the least length of time that the mirror is controllable to be held in the ON position. The length of time that each mirror is controlled to be held in an ON position is in turn controlled by multiple bit words.
FIG. 1D shows the “binary time periods” in the case of controlling the SLM by four-bit words. As shown in FIG. 1D, the time periods have relative values of 1, 2, 4, and 8 that in turn determine the relative intensity of light of each of the four bits, where “1” is the least significant bit (LSB) and “8” is the most significant bit. According to the PWM control mechanism, the minimum intensity of light that determines the resolution of the gray scale is a brightness controlled by using the “least significant bit” which holds the mirror at an ON position for the shortest controllable length of time.
For example, assuming n bits of gray scales, the frame time is divided into 2n−1 equal time periods. For a 16.7 milliseconds frame period and n-bit intensity values, the time period is 16.7/(2n−1) milliseconds
In recent years, projection apparatuses which use a laser light source as the light source, have been proposed in order to achieve a greater brightness and a broader gamut of color reproduction in the image display and a miniaturization of the projection device. When a laser light source is used as light source, however, there is a possibility of the “speckle effect” occurring when projecting an image with a high degree of coherence in the laser light. The speckle effect is a speckled pattern caused by different lights reflected diffusely at various points of a projection surface, interfering with one another in irregular phase relationships.
FIG. 2 is a diagram illustrating an example of a projection image from an observer's perspective when a speckle effect occurs. A familiar example of the speckle effect is commonly observed in the appearance of a glare in the spot where a laser light is projected on a wall using a laser pointer.
The methods for eliminating a speckle effect in the projection apparatus using a laser light source mainly include the following.
1. The method for changing the occurrence of the speckle effect by changing the condition of a diffuse reflection on a projection surface, thereby making the speckle effect inconspicuous.
Specifically, U.S. Pat. No. 5,272,473 discloses a method for oscillating a projection screen. This method, however, physically drives a gigantic screen and is therefore faced with the problems of high cost and high power consumption.
2. The method for reducing the coherency of a laser light.
Specific methods include:
(a) the method for causing the illumination light (i.e., laser light) from a laser light source to be reflected for a substantial number of times within an optical fiber. This method, however, lengthens the optical fiber and is therefore faced with limitations when miniaturizing the optical system.
(b) the method for dividing an illumination light path into a plurality thereof and changing the respective light path length, as disclosed in U.S. Pat. No. 6,249,381. This method, however, is faced with the problem in that it is difficult to make the optical system compact.
(c) the method for moving or rotating a diffuser placed in an illumination light path, as disclosed in U.S. Pat. Nos. 5,313,479, 6,594,090 and 6,874,893. This method, however, is faced with the problem that the usage efficiency of the laser light is reduced.
(d) the method for designing the generating frequency of a laser light to be as broad as possible (i.e., to have a “top hat” characteristic). This method, however, is faced with the problem that the design itself is technically very difficult.
The dither process or error diffusion method is a method for correcting a lack of gradation in an image. This is a method for artificially reproducing the gradation of one pixel on the basis of a plurality of pixels by utilizing the fact that the human eye has a low sensitivity to the fine part of an image, that is, the part with a high frequency. Therefore, an image displayed by applying, for example, a dither process in a projection apparatus appears totally different from an image represented by the original image data in terms of strictly observing it pixel by pixel, yet it can be viewed as the original image when viewing it from a distance so that the pixel size is not conspicuous.
FIG. 3A is a diagram exemplifying an image when it is displayed without applying a dither process; FIG. 3B is a diagram exemplifying an image when it is displayed by applying a dither process. As shown in the enlarged partial image 41 (FIG. 3A) and the enlarged partial image 42 (FIG. 3B), when the image is minutely viewed pixel by pixel, the image to which the dither process is applied is totally different from the original image to which a dither process is not applied. However, when the image is viewed from a distance so that the pixel size is not conspicuous, the image to which a dither process is applied appears similar to the original image, to which a dither process is not applied, as shown in the total image 43 (FIG. 3A) and the total image 44 (FIG. 3B).
FIG. 4 is a diagram describing an example of controlling a spatial light modulator (SLM) comprised in a projection apparatus when the image 43 shown in FIG. 3A is displayed. This control example exemplifies the case of controlling the individual pixel elements of the SLM by means of a PWM control, exemplifying the control for each frame period (T) of the pixel element corresponding to the pixels included in the partial image 45 within the image 43. Furthermore, the control exemplifies the case of reproducing the gray scale of the pixels included in the partial image 45 at the same level.
As shown in FIG. 4, according to the control example, each of the pixel elements corresponding to the pixels included in the partial image 45 is controlled under an ON state (noted as “turned ON” hereinafter for simplicity) during the period t2 within one frame period, while it is controlled under an OFF state (noted as “turned OFF” hereinafter for simplicity) during the other periods t1 and t3 within the aforementioned one frame period. Furthermore, such a control during one frame period is repeated. As a result, each of the pixel elements corresponding to the pixels included in the partial image 45 is turned OFF in the period t1, then turned ON in the period t2 and turned OFF in the period t3 during one frame period, and thereby the gradation of the partial image 45 per one frame period is obtained. Note that FIG. 4 shows the control example of four pixel elements (1, 2, 3 and 4) corresponding to the four pixels (pixels 1, 2, 3 and 4) within the partial image 45 as representatives. Furthermore, the figure expresses, by way of darkness, the gradation of each pixel included in the partial image 45 in each of the periods t1, t2 and t3.
Furthermore, when the image 44 shown in FIG. 3B is displayed, the control for the SLM comprised in the projection apparatus can also be carried out in a similar fashion to the control example shown in FIG. 4, on the basis of image data after a dither process is applied. Even if a dither process is applied for correcting a lack of gradation in an image, the above described speckle effect may occur if a laser light source is used as the light source of the projection apparatus and if the image to be displayed is monotonous, such that the gradation of individual pixels are constant, as represented by the image 44.